1. Field of Invention
The present invention relates to an electro-optical panel having two transistor elements per pixel, and to an electronic device using the electro-optical panel.
2. Description of Related Art
Currently, a conventional liquid-crystal display panel include a component substrate, an opposing substrate, and a liquid crystal held between these substrates. A plurality of data lines and a plurality of scanning lines are formed in an image display area of the component substrate, and a thin-film transistor (TFT) is provided in each of the pixels arranged in a matrix in such a manner as to correspond to the intersections thereof.
Various types of pixel circuit construction have been proposed, and in one such pixel construction, a P-channel TFT and an N-channel TFT are used in combination. FIG. 26 shows an exemplary circuit diagram having a circuit corresponding to one pixel of a component substrate used in a conventional liquid-crystal device. In this figure, the source electrodes of a P-channel TFT 1 and an N-channel TFT 2 are connected to a data line 6, whereas the gate electrode of the P-channel TFT 1 is connected to a scanning line 5a and the gate electrode of the N-channel TFT 2 is connected to a scanning line 5b. 
Also, the drain electrodes of the P-channel TFT 1 and the N-channel TFT 2 are connected to a pixel electrode 3, and the drain electrode of the N-channel TFT 2 is connected to a holding capacitor 4. Here, the pixel electrode 3, the common electrode formed on the opposing substrate, and the liquid crystal forms a liquid-crystal capacitor.
In such a pixel construction, an image signal supplied to the data line 6 can be written into a liquid-crystal capacitor 7 and the holding capacitor 4 when the P-channel TFT 1 and the N-channel TFT 2 are turned on. Then, when the P-channel TFT 1 and the N-channel TFT 2 are turned off, the voltage written into the liquid-crystal capacitor 7 and the holding capacitor 4 is held. Since the transmittance of the liquid crystal varies according to the applied voltage, it become possible to produce a gray-scale display.
Here, the reason the holding capacitor 4 is provided in addition to the liquid-crystal capacitor 7 is that a decrease in the applied voltage to the liquid crystal due to off-leakage of the P-channel TFT 1 and the N-channel TFT 2 is prevented and crosstalk in the vertical direction is prevented.
In the above-described pixel construction, the writing path of the image signal with respect to the holding capacitor 4 differs between a case in which the image signal passes through the P-channel TFT 1 and a case in which the image signal passes through the N-channel TFT 2. In other words, as shown in FIGS. 27 and 28, when the image signal passes through the P-channel TFT 1, the image signal is written into the holding capacitor 4 via the pixel electrode 3, whereas when the image signal passes through the N-channel TFT 2, the image signal is directly written into the holding capacitor 4 without passing through the pixel electrode 3.
FIG. 27 shows an exemplary circuit diagram having an equivalent circuit for the case in which an image signal is written into the holding capacitor 4 via the N-channel TFT 2. FIG. 28 shows a circuit diagram having an equivalent circuit for the case in which an image signal is written into the holding capacitor 4 via the P-channel TFT 1. In these figures, reference character Ron denotes an ON resistance of the P-channel TFT 1 and the N-channel TFT 2. Reference character Rito denotes the equivalent resistance of the pixel electrode 3. Reference character Ch denotes the capacitance value of the holding capacitor 4.
As is clear from these figures, the time constant when the image signal passes through the N-channel TFT 2 is xe2x80x9cRonxc2x7Chxe2x80x9d, whereas the time constant when the image signal passes through the P-channel TFT 1 is xe2x80x9c(Ron+Rito)xc2x7Chxe2x80x9d. Here, the equivalent resistance Rito of a pixel electrode 8 is greater than the ON resistance Ron.
Therefore, when the image signal is written from the P-channel TFT 1, the time constant becomes greater than in the case in which the image signal is written from the N-channel TFT 2. For this reason, when the difference between the voltage of the holding capacitor 4 and the voltage of the image signal is large, the image signal cannot be sufficiently written when it passes through the P-channel TFT 1, presenting the problem that a large contrast ratio cannot be obtained.
In particular, when a high-resolution image is to be displayed, the number of the scanning lines 5a and 5b, and the data lines 6 is increased. The greater the number of the scanning lines 5a and 5b and the data lines 6 is increased, the shorter the selection period of the scanning lines 5a and 5b and the data lines 6 becomes. As a result, insufficient writing due to the difference in the time constants becomes a serious problem.
The present invention has been made in view of the above-described circumstances. An object of the present invention is to provide an electro-optical panel which is capable of effectively exhibiting the features of the construction in which two transistor elements are used per pixel, and an electronic device using the electro-optical panel.
To achieve the above-mentioned object, the electro-optical panel of the present invention can include a component substrate, an opposing substrate, and an electro-optical material held between the component substrate and the opposing substrate. The component substrate can further include a plurality of scanning lines which are provided as pairs, a plurality of data lines, pixel electrodes which are arranged in a matrix in such a manner as to correspond to the intersections of the scanning lines and the data lines and which are each arranged between one of the scanning lines and the other of the scanning lines which are provided as a pair, a first transistor element in which the gate electrode is connected to one of the scanning lines which are provided as a pair, the data lines are connected to the source electrode, and the pixel electrode is connected to the drain electrode. The component substrate can further include a second transistor element in which the other one of the scanning lines in the pair is connected to the gate electrode, the data lines are connected to the source electrode, and the pixel electrode is connected to the drain electrode, a capacitance element which is connected to the drain electrode of the second transistor element and wiring which connects the drain electrode of the first transistor element to the drain electrode of the second transistor element.
According to the present invention, since wiring for connecting the drain electrode of the first transistor element to the drain electrode of the second transistor element is provided, the drain electrode of the first transistor element is connected to the drain electrode of the second transistor element through a different path from the pixel electrode. Therefore, since the equivalent resistance between the two drain electrodes is smaller than that in the case where wiring is not provided, the time constant when a signal is written into the holding capacitor via the first transistor element can be decreased.
Here, the resistance of the wiring is preferably smaller than the equivalent resistance of the pixel electrode which connects the drain electrode of the first transistor element to the drain electrode of the second transistor element. Furthermore, from the viewpoint of a lower resistance, it is preferable for the wiring that a high-melting point material, such as aluminum, silver, or chromium, be used. By decreasing the resistance of the wiring in this manner, the time constant when a signal is written into the holding capacitor via the first transistor element can be decreased even more. When the selection period of the scanning lines and the data lines is short, the image signal supplied to the data lines must be written into the holding capacitor in a short time. If wiring which causes the section between drain electrodes to be short-circuited is provided in the manner described above, it is possible to decrease the time constant when the image signal is written into the holding capacitor via the first transistor element.
As a result, since the image signal can be sufficiently written into the holding capacitor in a short time, a high contrast ratio can be obtained, and a vivid image can be displayed. Also, even if the number of scanning lines and data lines is increased and the selection period becomes shorter, since the image signal can be reliably written into the holding capacitor in such a selection period, it becomes possible to display a high-resolution image with high quality.
Furthermore, display variations which occur due to the difference in time constants can be greatly reduced. As a result of the writing into the holding capacitor becoming easier, the signal amplitude of the image signal to be supplied to the data lines can be decreased. Also, as a consequence of this, the amplitude of the image signal can be decreased. As a result, the power-supply voltage of the driving circuit can be decreased, allowing the power consumption to be reduced.
Also, a common electrode and a lattice-shaped black matrix may be formed on the opposing substrate, and the wiring may be arranged in such a manner as to overlap with the black matrix. Since light is not transmitted through the wiring portion, if the wiring is provided simply, the aperture ratio will be decreased. If the wiring is arranged so as to overlap with the black matrix as in the present invention, the writing time constant can be decreased without reducing the aperture ratio.
Next, the electro-optical panel of the present invention can include a component substrate, an opposing substrate, and an electro-optical material held between the component substrate and the opposing substrate The component substrate can further include a plurality of scanning lines which are provided as pairs, a plurality of data lines, pixel electrodes which are arranged in a matrix in such a manner as to correspond to the intersections of the scanning lines and the data lines and which are each arranged between one of the scanning lines which are provided as a pair and the other of the scanning lines, a first transistor element in which one of the scanning lines in the pair is connected to the gate electrode, and the data lines are connected to the source electrode. The component substrate can further include a second transistor element in which the other of the scanning lines provided in the pair is connected to the gate electrode, and the data lines are connected to the source electrode, and wiring which connects the drain electrode of the first transistor element to the drain electrode of the second transistor element, wherein either one of the drain electrode of the first transistor element and the drain electrode of the second transistor element is connected to the pixel electrode.
According to the present invention, the drain electrode of either one of the first transistor element and the second transistor element is connected to a pixel electrode. Since wiring for connecting each drain electrode is provided separately, it becomes possible to apply a voltage corresponding to an image signal to the pixel electrode. Furthermore, the connection between the drain electrode and the pixel electrode is required at only one place.
Here, the component substrate may comprise a capacitance element which is connected to the drain electrode of the second transistor element. In this case, the image signal is written into the holding capacitor via the wiring.
In the above-described electro-optical panel of the present invention, preferably, the first and second transistor elements comprise a polysilicon layer formed of a source region, a gate region, and a drain region, and a gate insulating film formed on the polysilicon layer, the drain electrode is connected to the drain region via a first contact hole formed in the gate insulating film, the pixel electrode and the drain electrode are connected to each other via a second contact hole, and the resistance of the wiring is smaller than the equivalent resistance of the first contact hole or the equivalent resistance of the second contact hole. According to the present invention, by decreasing the resistance of the wiring, the time constant when a signal is written into the holding capacitor via the first transistor element can be decreased even more.
Preferably, the first and second transistor elements comprise a polysilicon layer formed of a source region, a gate region, and a drain region, and a gate insulating film formed on the polysilicon layer, the drain electrode is connected to the drain region via a first contact hole formed in the gate insulating film, the pixel electrode and the drain electrode are connected to each other via a second contact hole, a common electrode and a lattice-shaped black matrix are formed on the opposing substrate, and the second contact hole is provided in such a manner as to overlap with the black matrix. Since the contact state of the second contact hole with the electro-optical material differs from that of the pixel electrode, for the region concerned, the state of the electric field applied to the electro-optical material differs from that of the pixel electrode. However, according to the present invention, since the second contact hole is covered with the black matrix, the region concerned, having a different brightness, can be concealed from the view.
The first transistor element may comprise a P-type thin-film transistor element, and the second transistor element may comprise an N-type thin-film transistor element. Conversely, the first transistor element may comprise an N-type thin-film transistor element, and the second transistor element may comprise a P-type thin-film transistor element.
Furthermore, preferably, the capacitance element is formed between a capacitor line formed in proximity to the other scanning line and the drain region of the second transistor element.
In order to construct a holding capacitor, two opposing electrodes are required. In this example, since one of the electrodes is also used for the drain region of the second transistor element, there is no need to provide a contact for connecting one of the electrodes to the drain region. Consequently, the manufacturing process can be simplified, and an area for providing the holding capacitor can be reduced to improve the aperture ratio.
In the present invention, the pixel electrode can be formed of a light-transmitting conductive film, for example.
Also, in the present invention, the pixel electrode may be formed of a light-transmitting conductive film, may have the electro-optical material side positioned on one of the sides thereof, and may comprise a reflection layer which reflects incident light on the other side. In this case, the wiring is formed on the side opposite to the pixel electrode with respect to the reflection layer. Incident light is not transmitted through the space on the side opposite to the pixel electrode of the reflection layer, and by providing wiring in this space, the writing time constant of the image signal can be decreased without decreasing the aperture ratio.
Here, in a case where an opening through which light is transmitted is provided in a portion of the reflection layer, the wiring is preferably arranged in such a manner as not to overlap with the opening. Since the opening causes light to be transmitted therethrough, if wiring is provided in this region, the aperture ratio is decreased. However, according to the present invention, the aperture ratio is not decreased.
In the present invention, the pixel electrode may be formed of a light-reflecting conductive film.
Furthermore, the electro-optical panel of the present invention may be such that a scanning-line driving circuit for selecting the scanning lines, which form a pair, in sequence, and a data-line driving circuit for supplying an image signal to each of the data lines are formed on the component substrate.
Next, the electronic device of the present invention comprises the above-described electro-optical panel, and examples thereof include a viewfinder used in a video camera, a portable phone, a notebook-sized computer, and a video projector.